1. Field of the Invention
The present invention relates to an image pick-up apparatus and, more particularly, to white balance control of the image pick-up apparatus utilizing an image pick-up device having a plurality of pixels that are different in sensitivity.
2. Description of the Related Art
In an image pick-up apparatus such as a digital still camera, white balance control is performed to allow color reproduction with high fidelity to the object. In the case of common white balance control, gains for R and B color components among color information of three colors, i.e., R, G, and B of an image which has been acquired are adjusted on the basis of the G color component such that the levels of the colors become equal.
First white balance controlling functions provided in image pick-up apparatus include automatic white balancing that is an automatic white balance adjustment performed by the apparatus through determination of the scene being photographed according to a predetermined control procedure and, in addition, so-called custom white balancing that allows a user to perform a specific white balance adjustment for some purpose. For example, custom white balancing is carried out by performing a certain operation such as pressing a button while photographing an object such as white paper to control gains such that the levels of three colors, i.e., R, G, and B of the object image are made equal to achieve a white balance. The custom white balancing allows white balance control to be properly performed depending on photographic conditions such as a particular light source.
In an example of conventional white balance control, average values of color information of three colors, i.e., R, G, and B of an image which has been acquired are calculated; gains for the R and B color information relative to the G color information are calculated such that the calculated average values become equal; the calculated gains for the color information are compared with gains for the color information which has been calculated in advance; and, when comparison results indicate that they are substantially the same, white balance of the acquired image is changed based on the average values and the gains of color information thus calculated, thereby preventing inadequate changes of the color of the image (see JP-A-9-294273, for example).
Alternatively, a strobe is made to emit light by operating predetermined operating means; an image pick-up signal is acquired from an object at second timing after first timing at which the strobe emits light; and white balance adjustment is performed based on the image pick-up signal thus acquired. This allows white balance to be set in accordance with the color temperature of an actual light source even when a photograph is taken using a strobe (see JP-A-5-111045, for example).
FIG. 6 is a flow chart showing an example of a gain setting process included in custom white balance control performed as described above. First, a button or the like is operated to instruct the commencement of custom white balance setting, and a photographing sequence is performed to acquire an image pick-up signal (step S51). Next, integrated values of three color components, i.e., R, G, and B of the acquired image pick-up signal are calculated (step S52), and it is judged whether the calculated values are in a specified range of signal levels (step S53). When the integrated values are in the predetermined range of levels, gain values for white balance adjustment are calculated such that the levels of the three color components, i.e., R, G, and B become equal (step S54), and the gain values for the colors thus calculated are written in a memory as gain values for custom white balancing (step S55). When the integrated values are out of the predetermined range of levels, white balance is set at “fine position” (step S56), and an error indication is given (step S57).
When a photograph is taken with such a setting of custom white balance, white balance adjustment is performed on an image pick-up signal acquired through a photographing sequence using the gain values for custom white balancing held in the memory.
Second white balance controlling functions provided in image pick-up apparatus include automatic white balancing that is an automatic white balance adjustment performed by the apparatus through determination of the scene being photographed according to a predetermined control procedure and, in addition, manual white balancing that is white balance adjustment performed by a user through manual selection of a white balance position depending on the scene being photographed or the light source.
In an example of conventional automatic white balancing, the scene being photographed and the light source at the time of photographing are determined from color information of the image signals photographed, and white balance adjustment is performed using a predetermined gain (see JP-A-6-165189 and JP-A-2000-299876, for example). In an example of conventional manual white balancing, the photographer selects a white balance position such as “fine”, “cloudy”, “electric bulb”, or “fluorescent lamp” position that is set for each scene to be photographed or each light source, and white balance adjustment is performed using a gain associated with the white balance position thus selected (see JP-A-2000-299876 and JP-A-11-187412, for example).
JP-A-9-294273 (pp. 3-4 and FIG. 2), JP-A-5-111045 (pp. 3-5 and FIG. 1), JP-A-6-165189 (pp. 3-4 and FIG. 1), JP-A-2000-299876 (pp. 5-7 and FIG. 1) and JP-A-11-187412 (pp. 3-5 and FIG. 1) are known as related art.
For example, let us assume an image pick-up apparatus such as a digital still camera in which a solid-state image pick-up device has both of pixels having high sensitivity and pixels having low sensitivity and in which a high sensitivity image picked up by the high sensitivity pixels and a low sensitivity image picked up by the low sensitivity pixels are synthesized and output as data of one image to achieve a wide dynamic range. In such an image pick-up apparatus, the high sensitivity pixels and the low sensitivity pixels may have different spectral sensitivity characteristics, and the high sensitivity pixels and the low sensitivity pixels may be different from each other in the ratio between output signal levels of R, G, and B color components even when a photograph is taken under the same photographic conditions. As a result, color balance of an image signal obtained by synthesizing the output of the high sensitivity pixels and the output of the low sensitivity pixels can be lost when the above-described custom white balance control is performed.